A polyimide using, as a tetracarboxylic acid component, 3,3′,4,4′,-biphenyltetracarboxylic dianhydride (hereinafter, simply referred to as s-BPDA) and/or pyromellitic dianhydride (hereinafter, simply referred to as PMDA) and using, as a diamine component, p-phenylenediamine (hereinafter, simply referred to as PPD) is excellent in heat resistance, mechanical strength, dimensional stability and the like, and therefore is being used for various applications.
However, when obtaining a laminate having a substrate and a polyimide film by coating the substrate with a polyamic acid solution composition having the above-described chemical formulation and heat-treating the coating, bubbling or bulging may occur, and a problem arises in the formability. Therefore, a polyimide film having this chemical formulation is usually obtained by a method of coating a polyamic acid solution composition on a substrate, heating/drying the obtained coating to form a self-supporting film, separating the self-supporting film from the substrate, and further subjecting the film to a heating imidation treatment.
On the other hand, when obtaining a laminate having a substrate and a polyimide film by coating a substrate with a polyamic solution composition having the above-described chemical formulation and heat-treating the coating, in addition to the problem in formability that bubbling or bulging may occur, the obtained polyimide film is may poor in characteristics such as heat resistance. Above all, in the case of using a polyamic acid solution composition composed of a polyamic acid having a relatively low molecular weight, which is easy to handle and facilitates suitable formation of a coating film, there is a problem that the obtained polyimide film has reduced heat resistance.
Patent Document 1 describes a production process of a polyimide film, where a specific component is employed for a part of the above-described chemical formulation and the formability is thereby improved while suppressing reduction in the characteristics.
Patent Document 2 describes a production process of a polyimide film, where a solution of a polyamic acid solution composition having the above-described chemical formulation is combined with a specific organic polar solvent to obtain a mixed solvent and the formability is thereby improved, making it possible to obtain a polyimide film with excellent characteristics.
In these documents, as for the solvent, an organic polar solvent capable of dissolving a polyamic acid is suitably used, but a mixed solvent of such a solvent and a solvent azeotropic with water is not described.
Patent Document 3 suggests that for the purpose of adjusting the degree of imide ring closure, it is also possible to employ one of the methods where heating is performed, for example, by allowing a substance having dehydration action, such as molecular sieve, to be present in varnish, is performed in the presence of an azeotropic solvent such as toluene, or is performed under reduced pressure and water generated along with imidation is thereby removed to the outside of the system, or employ two or more of these methods in combination.
Patent Document 4 describes a technique of coating a polyimide precursor on a supporting substrate (substrate) such as glass, subjecting the coating to a heating imidation treatment to form a heat-resistant base substrate (polyimide film), stacking a transparent electrode layer, an amorphous silicon layer, a back plate layer and the like thereon, further forming a protective layer, and performing separation between the supporting substrate and the heat-resistant base substrate to obtain a thin-film solar cell having flexibility, and suggests a problem that if the heat treatment at the formation of the heat-resistant base substrate (polyimide film) is insufficient, an outgas may be generated when forming the amorphous silicone layer.
In order to solve the problem above, Patent Document 4 has proposed to form the heat-resistant base substrate by two layers of a polyimide film and a release resin layer (polyamic acid) for bonding the polyimide film. However, formation two layers is not economical because of increase in the steps and also requires a complicated control. Furthermore, the release resin layer needs to be formed of a polyamic acid, and therefore the problem similar to that encountered at the formation of a polyimide film is not satisfactorily solved.